With the compact disk (hereinafter referred to as CD), that may be called the first generation optical disk, information is recorded on or reproduced from (hereinafter expressed as record/reproduce) an optical disk having a protective layer 1.2 mm in thickness by using an objective lens with a numerical aperture from 0.45 to 0.5 and infrared rays of wavelength 780 nm. In this specification, the term protective layer means a transparent medium disposed between a surface whereon a light beam incident on the optical disk strikes and an information recording surface. In a digital versatile disk (hereinafter referred to as DVD), the second generation optical disk, information is recorded/reproduced on/from the optical disk having a protective layer 0.6 mm in thickness by using an objective lens with a numerical aperture of 0.6 and red light of wavelength 655 nm. In a Blue Ray disk (hereinafter referred to as BD), the third generation optical disk, information is recorded/reproduced on/from the optical disk having a protective layer 0.1 mm or 0.075 mm in thickness by using an objective lens with a numerical aperture of 0.85 and blue light of wavelength 405 nm.
The third generation optical disk, which uses the blue laser beam of short wavelength and an optical system having a large numerical aperture, achieves unprecedentedly high recording density and is expected to proliferate. In the meantime there are needs for a compact and inexpensive optical disk device of BD specification, which is a high-density optical disk that can record/reproduce data stored in a DVD or a CD, for the purpose of preserving and utilizing the information properties stored in the old disks. For this purpose, an optical pickup that can record/reproduce information with three wavelengths on/from optical disks having protective layers of different thicknesses with a single objective lens has been developed. With this optical pickup, spherical aberration due to the difference in thickness of the protective layer is compensated for mainly by means of a hologram and the aperture is restricted by using an optical filter or a diffraction element.
There have been disclosed constitutions for recording/reproducing information on/from optical disks of different types by restricting the aperture for light beams of different wavelengths. This constitution will be described with reference to FIG. 17 that schematically shows the constitution of an example of the optical pickup of the prior art. In FIG. 17, a light beam 61 having a wavelength of 405 nm emitted by a blue laser 60 is collimated by a collimator lens 62 and passes through a polarization beam splitter 63 and a dichroic prism 64, and is reflected on a mirror 65. The reflected beam is then circularly polarized by a quarter wavelength plate 66, passes through a wavelength selective aperture 67 and is focused by an objective lens 68 with a numerical aperture of NA1 on an optical disk 51 having a protective layer 0.1 mm in thickness. Reflected light from the optical disk 51 passes through the objective lens 68, the wavelength selective aperture 67 and the quarter wavelength plate 66 so as to be linearly polarized perpendicular to that in the outward path, and is reflected by the polarization beam splitter 63. The reflected light is focused by a detector lens 69 so as to enter a light receiving surface of a photodetector 71. Output of the photodetector 71 is processed to obtain information signals and control signals.
A hologram unit 72 is constituted from a red laser 72a, an infrared laser 72b, a hologram 72c and light receiving elements 72d, 72e which are integrated into the unit. A light beam 73 emitted by the red laser 72a is collimated by the collimator lens 74, and is reflected by the dichroic prism 64 and the mirror 65. The reflected beam is then circularly polarized by the quarter wavelength plate 66, passes through the wavelength selective aperture 67 being subjected to aperture restriction and is focused by the objective lens 68 with a numerical aperture of NA2 on an optical disk 52 having a protective layer 0.6 mm in thickness. Reflected light from the optical disk 52 passes through the objective lens 68, the wavelength selective aperture 67 and the quarter wavelength plate 66 so as to be linearly polarized perpendicular to that in the outward path, and is reflected by the dichroic prism 64. The reflected light is focused by the collimator lens 74, diffracted by the hologram 72c and enters the light receiving element 72d. Output of the light receiving element 72d is processed to obtain information signals and control signals.
A light beam 75 emitted by the infrared laser 72b is collimated by the collimator lens 74, and is reflected by the dichroic prism 64 and the mirror 65. The reflected beam is then circularly polarized by the quarter wavelength plate 66, passes through the wavelength selective aperture 67 being subjected to aperture restriction and is focused by the objective lens 68 with a numerical aperture of NA3 on an optical disk 53 having a protective layer 1.2 mm in thickness. Reflected light from the optical disk 53 passes through the objective lens 68, the wavelength selective aperture 67 and the quarter wavelength plate 66 so as to be linearly polarized perpendicular to that in the outward path, and is reflected by the dichroic prism 64. The reflected light is focused by the collimator lens 74, diffracted by the hologram 72c and enters the light receiving element 72e. Output of the light receiving element 72e is processed to obtain information signals and control signals.
Spherical aberration due to the difference in thickness of the protective layer between the disks is compensated for by spherical aberration compensating means (not shown).
The wavelength selective aperture 67 is shown in FIG. 18. In FIG. 18, a central area 67a of the wavelength selective aperture 67 is a region corresponding to the numerical aperture NA3, where a multi-layer optical film is formed that transmits the light beams 61, 73 and 75. An annular area 67b is a region corresponding to a numerical aperture ranging from NA3 to NA2, where a multi-layer optical film is formed that transmits the light beam 61 and the light beam 73 and reflects the light beam 75. A peripheral area 67c is a region having a numerical aperture larger than NA2, and has a multi-layer optical film formed therein that transmits the light beam 61 and reflects the light beam 73 and the light beam 75. As a result, the light beam 75 is focused with the numerical aperture NA3 on the optical disk 53, and the light beam 73 is focused with the numerical aperture NA2 on the optical disk 52. It is necessary to form the multi-layer optical film in the central area 67a so as to match the phases of the light beams 61, 73 that transmit therethrough and the phases of the light beams 61, 73 that transmit through the annular area 67b and the peripheral area 67c (for example, Japanese Unexamined Patent Publication (Kokai) No. 2003-255221 (pp. 12-13, FIG. 10)).
A second example of the prior art has such a constitution of an optical pickup that restricts the aperture by using a diffraction element. This constitution will be described with reference to FIG. 19, which shows the schematic constitution of an example of an optical pickup of the prior art. A light beam 81 having a wavelength of 405 nm emitted by a blue laser 80 is collimated by a collimator lens 82, passes through a polarization beam splitter 83, a beam expander 84, a polarization beam splitter 85 and a diffraction optical element 86, and is focused by an objective lens 87 with a numerical aperture of NA1 on the optical disk 51 having a protective layer 0.1 mm in thickness. Reflected light from the optical disk 51 passes again through the objective lens 87, the diffraction optical element 86, the polarization beam splitter 85 and the beam expander 84, and is reflected by the polarization beam splitter 83. The reflected light is astigmatized by a detector lens 88 and is focused on the light receiving surface of a photodetector 89. Output of the photodetector 89 is processed to obtain information signals and control signals.
A light beam 91 having a wavelength of 655 nm emitted by a red laser 90 passes through polarization beam splitters 92, 93, is collimated by a collimator lens 94, and is reflected by the polarization beam splitter 85. The reflected light beam, of which the diameter is restricted by the diffraction optical element 86, is focused by an objective lens 87 with a numerical aperture of NA2 on the optical disk 52 having a protective layer 0.6 mm in thickness. Reflected light from the optical disk 52 passes again through the objective lens 87 and the diffraction optical element 86, and is reflected by the polarization beam splitter 85. The reflected light is focused by the collimator lens 94, reflected by the polarization beam splitter 85, astigmatized by a detector lens 95 and is focused on the light receiving surface of a photodetector 96. Output of the photodetector 96 is processed to obtain information signals and control signals.
A light beam 98 having a wavelength of 780 nm emitted by an infrared laser 97 is reflected by the polarization beam splitter 92, passes through the polarization beam splitter 93, collimated by the collimator lens 94 to become a parallel beam, and is reflected by the polarization beam splitter 85. The reflected beam, of which the diameter is restricted by the diffraction optical element 86, is focused by an objective lens 87 with a numerical aperture of NA3 on the optical disk 53 having a protective layer 1.2 mm in thickness. Reflected light from the optical disk 53 passes again through the objective lens 87 and the diffraction optical element 86, and is reflected by the polarization beam splitter 85. The reflected light is focused by the collimator lens 94, reflected by the polarization beam splitter 93, astigmatized by the detector lens 95 and is focused on the light receiving surface of the photodetector 96. Output of the photodetector 96 is processed to obtain information signals and control signals.
Spherical aberration due to the difference in thickness of the protective layer between the disks is compensated for by a hologram provided separately.
The diffraction optical element 86 is shown in FIG. 20A and FIG. 20B. In FIG. 20A, the diffraction optical element 86 comprises an area 861 that does not have a diffraction structure in a range corresponding to the numerical aperture of NA3, an area 862 that has a diffraction structure 86a in a range corresponding to a numerical aperture from NA3 to NA2, and an area 863 that has a diffraction structure 86b formed on the outside within a range corresponding to the numerical aperture NA2, the diffraction structures being formed as a stepwise structure as shown in FIG. 20B. The diffraction structure 86a is formed such that difference in optical path length approximately equal to integer times the wavelengths λ1 and λ2 is generated by one step of the stepwise structure, so that the light beams 81, 91 are transmitted without being diffracted while the light beam 98 is diffracted to become unnecessary light. The diffraction structure 86b is formed such that difference in optical path length approximately equal to integer times the wavelength λ1 is generated by one step of the stepwise structure, so that the light beam 81 is transmitted without being diffracted while the light beams 91, 98 are diffracted to become unnecessary light. As a result, the light beam 98 is focused with the numerical aperture NA3 on the optical disk 53, and the light beam 91 is focused with the numerical aperture NA2 on the optical disk 52 (for example, Japanese Unexamined Patent Publication (Kokai) No. 2005-259332 (pp. 20-24, FIGS. 1, 2)).
In the first example of the prior art, since the aperture restriction is applied for a DVD and a CD by the wavelength selective aperture 67, it is necessary to provide multi-layer optical films of three types on one surface. The multi-layer optical film is formed from a dielectric material having a high refractive index, for example, Ta2O5 that has high transmissivity for blue light and SiO2 having high refractive index, stacked one on another. The multi-layer optical film can be formed by applying a photoresist to a metal film formed by vapor deposition so as to mask portions that would become the annular area 67b and the peripheral area 67c, removing the metal film from the central area 67a by etching, and the multi-layer optical film that transmits the light beam 61, the light beam 73 and the light beam 75 is formed by vapor deposition. Then the metal film and the multi-layer optical film are removed from the annular area 67b and the peripheral area 67c by lift-off, so as to complete the multi-layer optical film for the central area 67a. Then after forming the metal film by vapor deposition, a photoresist is applied to mask the portions of the central area 67a and the peripheral area 67c, the metal film is removed from the annular area 67b by etching, and the multi-layer optical film that transmits the light beam 61 and the light beam 73 and reflects the light beam 75 is formed by vapor deposition. Then the metal film and the multi-layer optical film are removed from the central area 67a and the peripheral area 67c by lift-off, so as to complete the multi-layer optical film for the annular area 67b. Last, the multi-layer optical film that transmits the light beam 61 and reflects the light beam 73 and the light beam 75 is formed by a similar process.
The wavelength selective aperture that employs the multi-layer optical film requires a complicated manufacturing process as described above and high production cost, since the steps of vapor deposition of the metal film, masking, vapor deposition of the multi-layer optical film and lifting off are repeated three times.
In the second example of the prior art, aperture restriction is applied for a DVD and a CD by the diffraction optical element 86. The light beam 98 that passes through the region 862 is diffracted by the diffraction structure 86a so as not to converge on the information recording surface of the optical disk 53, and the light beams 91, 98 that pass through the region 863 are diffracted by the diffraction structure 86b so as not to converge on the information recording surfaces of the optical disks 52, 53. This structure is capable of restricting the aperture for the light beam that converges on the information recording surface of the optical disk, although there is such a problem that, when the diffracted beam reflecting on the optical disk is diffracted again by the diffraction aperture element 86, it takes the same optical path as that of the light beam that has passed the region 861 and reflected by the optical disk 53, and is received by the photodetector 96. This problem will be described below with reference to FIG. 21.
FIG. 21 is a sectional view of the objective lens 87 and the diffraction optical element 86, showing the propagation of the light beam that converges on the optical disk 53. The light beam 98 that passes the region 862 is diffracted by the diffraction structure 86a and, for example, a +1st order diffracted light takes an optical path as shown in the drawing to reach the optical disk 53 and is reflected by the information recording surface of the optical disk 53. The +1st order diffracted light reflected by the optical disk 53 enters the diffraction structure 86a again so as to be diffracted. A −1st order diffracted light generated during the diffraction of the +1st order diffracted light is transmitted without being diffracted by the region 861, takes the same optical path as that of the light beam reflected by the optical disk 53, and enters the photodetector 96. When the −1st order diffracted light generated in the diffraction structure 86a is reflected by the optical disk 53 and is diffracted again so as to become the +1st order diffracted light, it enters the photodetector 96 similarly. This is not limited to the +1st order diffracted light and the −1st order diffracted light, and a +mth order diffraction beam and a −mth order diffraction beam, where m is an integer, similarly take the same optical path as that of the light beam that has transmitted the region 861, and enter the photodetector 96. This applies also to the light beams 91, 98 that are diffracted by the diffraction structure 86b. This means that, although the aperture can be restricted for the outward path, the aperture cannot be restricted for the return path, resulting in unnecessary light being superimposed on the reproduction signal and/or the control signal, thus deteriorating the quality of the signals.
The diffraction structures 86a, 86b, that are formed as a stepwise structure as shown in FIG. 20B, have the effect of decreasing the intensity of the diffracted light beam as it enters the photodetector by increasing the intensity of a particular diffraction beam, although the diffraction structure cannot be formed as a saw-tooth shape and the diffracted beam incident on the photodetector 96 cannot be avoided. Forming the diffraction structures as a stepwise structure also results in deeper grooves which increase the transmission loss due to configuration error and the diffraction loss.